The present invention relates generally to the field of microprocessors which are capable of processing at least two program instructions at the same time.
Modern microprocessors, including superscalar microprocessors, have improved performance due to the capability of processing at least two program instructions at the same time. This capability arises from having a first group of execution units which can receive a program instruction for execution, and a second group of execution units which can receive a second program instruction for execution.
FIG. 1 shows a typical microprocessor of the prior art which uses a dual issue mechanism wherein two program instructions may be issued to two groups of execution units. The register file 30 and the issue control and bypass control unit 12 support the issuance of two instructions, one instruction going to the execution units 14, 16, and 18 (issue left) and another instruction going to the execution units 20, 22, and 24 (issue right). Within each group of execution units, there are three specialized functional units. In particular, the execution units on the issue left side include a floating point division execution unit 14, a floating point multiplier execution unit 16, and an ALU execution unit 18. In the group of execution units on the issue right side, there is a floating point adder execution unit 20, an ALU execution unit 22, and an integer multiplier unit 24. Each of these execution units is coupled to the issue control and bypass control unit 12 by a bi-directional link which provides instructions from the issue control unit 12 to the particular execution unit and which provides a signal indicating the execution unit is busy to the issue control and bypass control unit 12. In this manner, the issue control and bypass control unit 12 can determine the status of each execution unit (e.g. is the particular execution unit busy executing an instruction previously provided?) and can provide instructions for execution if the particular execution unit is not busy. These links are shown as 31A-31F in FIG. 1. The issue control and bypass unit 12 is coupled to an instruction cache 10 through a bus 11. It will be appreciated that the issue control unit 12 provides read commands to the instruction cache 10 to cause the instruction cache 10 to deliver one or two instructions at a time to the issue control unit 12.
Each execution unit within a group of execution units is coupled to an output of a multiplexer in order to receive operands which are processed according to the instruction being executed in the particular execution unit. These operands are received from either the register file 30 or from a bypass pathway in which an output from a prior executed instruction is used as an operand for a current instruction. The multiplexer 26 receives an output 30b from the register file and also receives an output from each of the six execution units and provides a selected output to the three execution units 14, 16, and 18 in the issue left group of execution units. The multiplexer 28 receives an output 30a from the register file 30 and also receives outputs from each of the six execution units, and provides an output which is selected by the control select line 15. This output is provided to the three execution units 20, 22, and 24 in the issue right execution group. The six outputs 32a, 32b, 32c, 32d, 32e, and 32f from the six execution units 14, 16, 18, 20, 22, and 24 are provided to both multiplexers 26 and 28 and also provided to the register file 30 as inputs to the register file 30. It will be appreciated that the register file 30 may be configured to provide dual port reads such that operand outputs 30a and 30b can be provided based upon the addresses provided over address bus 17 from the control unit 12. Moreover, the register file 30 may support multiple writes, such as six multiple write ports from the six outputs. It will also be appreciated that in typical operation of the microprocessor shown in FIG. 1, only two of the write ports will be active at once since normally only one result of a computation is provided from the issue left side and only one execution result is provided from the issue right side.
The operation of the microprocessor shown in FIG. 1 will now be described. The issue control unit 12 receives two instructions from the instruction cache 10. The issue control unit 12 then decodes each instruction to determine the resources or functions to be performed as required by the particular instruction. For example, if an instruction requires floating point division or floating point multiplication, then this instruction must be steered into the issue left group of execution units. Similarly, if a decoded instruction reveals that a floating point addition or integer multiplication is required by the instruction, then it must be issued to the issue right group of execution units. Thus, decoding in the issue control and bypass control unit 12 is required in order to determine whether an instruction goes to issue left or to issue right.
The issue control and bypass control unit 12 must also perform the resolution of execution unit conflicts before issuing an instruction. The following table shows an example of the stall logic in the issue control unit 12 in order to resolve execution unit conflicts. If there is an execution unit conflict indicated by a "X", then the issue control will stall the issue of the instruction.
TABLE A ______________________________________ Issue Instruction Instruction in Unit: Type FP Div FP Mult FP Add ALU 0 ALU 1 Int Mult ______________________________________ FP Div X FP Mult X FP Add X ALU 1 X X Int Mult X ______________________________________
For example, if the issue instruction is of the type "FP Div" (i.e., the instruction is for a floating point division), the instruction will stall if there is an instruction in the floating point division unit 14 which is currently being executed by the floating point execution unit 14.
The issue control and bypass unit 12 also stalls the issuance of instructions in order to resolve register conflicts.
TABLE B ______________________________________ Issue Instruction Instruction in Unit: Registers FP Div FP Mult FP Add ALU 0 ALU 1 Int Mult ______________________________________ Operand 1 X X X X X X Operand 2 X X X X X X Destination X X X X X X ______________________________________
If there is a register match between any of the registers indicated by "X", then the issue control unit 12 will stall the issue on the instruction. For example, if for a particular instruction which is yet to be issued, if the first operand for the instruction is to be stored in the same register as the destination register for a floating point division operation which is currently being executed, then the yet to be issued instruction will be stalled.
The issue control unit 12 also resolves resource conflicts at the register file 30 which arise because different instructions have different processor cycle times. This is shown by way of example in Table C below.
TABLE C ______________________________________ Cycle ALU 0 ALU 1 FP Mult FP Add ______________________________________ 1 -- -- Issue -- 2 -- -- Execute Issue 3 -- -- Execute Execute 4 Issue Issue Execute Execute 5 Write Result Write Result Write Result Write Result ______________________________________
As can be seen from Table C, in Cycle 5 there are four results that are produced. Unless there are four write ports into the register file which contains a plurality of registers, the issue of instructions for the ALU0 and ALU1 in Cycle 4 may need to be stalled.
As can be seen from the foregoing description, the issue control unit must perform a variety of control operations in order to resolve various conflicts and yet attempt to issue two program instructions substantially concurrently if possible. It will be appreciated that such control, such as the decoding of program instructions in order to steer the instruction into the appropriate group of execution units, requires considerable circuitry and also requires considerable time in designing such a control unit for this type of microprocessor.
In many circumstances, it will be desirable to provide a microprocessor which requires less complicated issue control.